The present invention relates to a structure of an internal thread portion for preventing looseness and a method of producing the same.
Conventionally, a self-locking nut which resists being loosened by vibrations or the like after tightened against a bolt has been used in a vehicle or the like.
For example, Japanese Patent Laid-Open Publication S55-30593 discloses such a self-locking nut of which an exit end portion is crimped from outside at a plurality of locations to form a plurality of (e.g., three) projections on the inner circumference of the internal thread.
In the nut on which crimping is performed locally at a plurality of locations from outside to partially deform the nut itself to form projections, however, a variation is prone to occur in the accuracy of dimension of the projections and the variation causes such defects as follows: A deficient projection will not provide a sufficient effect of preventing looseness. On the other hand, an excessive projection will cause excessive friction locally between the projections and the external thread portion of a bolt. Such excessive friction involves a large tightening torque, so that a seizure is prone to occur when the nut is tightened with a power wrench or the like.
The invention is therefore intended to provide a structure of an internal thread portion of a nut or the like for preventing looseness which structure has the required torque reduced adequately to resist seizure and which still has a satisfactory effect of preventing looseness, and to provide a method of producing the same.
In accordance with the invention, there is provided a structure of an internal thread portion for preventing looseness, wherein an exit end portion out of an entrance end portion and the exit end portion of the internal thread portion to be threadedly engaged with an external-thread member is taperingly contracted on the entire circumference thereof so as to have smaller minor diameters of internal thread than a middle portion to form a contracted internal thread portion having a conical section,
wherein ridges of the contracted internal thread portion are provided in the form of circumferentially continuing corrugation which has projections projecting toward the center of the contracted internal thread portion and which has depressions depressed away from the center of the contracted internal thread portion, and
wherein, with the external-thread member threadedly engaged with the internal thread portion, a plurality of projections in the corrugation of the ridges of the contracted internal thread portion press roots or flanks of the external-thread member and are thereby elastically compressed by the reaction, so as to clamp the external-thread member from outside.
In accordance with the invention, the exit end portion is taperingly contracted on the entire circumference thereof so as to have smaller minor diameters of internal thread than the middle portion to form the contracted internal thread portion having a conical section. Accordingly, there hardly occurs such a variation in the deformation of the internal thread portion as in the case that pressing force(s) for deformation are applied to part of the circumference of the portion. In addition, the ridges in the contracted internal thread portion are provided with circumferentially continuing corrugation, and the projections in the corrugation are in even contact with the entire circumference of the thread portion of an external-thread member. Consequently, local friction is reduced and a seizure is less prone to occur, in comparison with conventional press-contact performed by the entire circumferences of the ridges. Furthermore, the projections in the continuing corrugation exert substantially even clamping forces over the entire circumference, thus providing a satisfactory effect of preventing looseness.
When the projections of the ridges in the contracted internal thread portion and the root or flanks of the external-thread member exert pressing forces on each other, the projections of the ridges are elastically compressed. At this time, however, depressions on the both sides of the projections allow compressed bodies of the projections to readily escape toward both sides thereof (i.e., the compressive force readily diffuses into the depressions on both sides). This escape prevents a steep increase in torque in threaded engagement, while, after the completion of engagement, the press-contact between the plurality of projections and the root or flanks provides a good effect of preventing looseness. That is, the compatibility is achieved between a reduction in torque in threaded engagement and the fulfillment of the effect of preventing looseness.
The specific configuration of the corrugation in the contracted internal thread portion may be like, for example, corrugation with a series of arcs, triangles or trapezoids of which projections broaden toward their base ends from their tip ends in plan view, or inverted triangles or inverted trapezoids of which depressions broaden toward their tip ends from their base ends in plan view. In any of the configurations, the tip end of the projection has a smaller area of cross section than the base end and therefore more readily undergoes elastic deformation, which reduces the tightening torque. The configuration corrugated with arcs would reduce the contact area between the projections and the thread portion of the external-thread member, so as to enhance the effect of preventing seizure, and would improve the life of the die. The trapezoidal configuration would enhance the effect of preventing looseness.
It is preferable that the corrugation of the ridges in the contracted internal thread portion is formed continuously over a plurality of threads and that, with respect to adjoining ridges, the depressions and projections of the corrugation are formed respectively at locations corresponding to one another on straight lines which traverse the ridges and which are generally tapered. In this manner,, the machining of the corrugation can be readily performed.
More specifically, the number of the threads provided with the corrugation is preferably not less than two and not more than four, and the number of the projections is preferably set within the range from 6 to 20. This range is adequate to keep the balance between the effect of preventing a seizure and reducing the tightening torque and the effect of preventing looseness and to expand the life of the die to prevent the cost from increasing. Too small number of the columns of the projections would expand the life of the die but enhance local friction, thus increasing the probability of the occurrence of the seizure. On the other hand, too large number of the columns of the projections would not cause such a problem of friction but shorten the life of the die, thus causing a rise in cost. It is therefore preferable that the number is determined within the above range.
A method of producing a structure of an internal thread portion for preventing looseness in accordance with the invention comprises the steps of:
forming, in an end portion of a material for the internal thread portion which end is to be an exit end portion in threaded engagement with a mating external-thread member, an exit-side hole having corrugation continuing circumferentially on the inner circumference thereof;
forming a main hole which connects coaxially with the exit-side hole, before, after, or simultaneously with the above step;
cutting an internal thread continuously on both the exit-side hole and the main hole so that the corrugation in the exit end portion remains and extends continuously and circumferentially on ridges of the internal thread; and
taperingly contracting the exit end portion over the entire circumference thereof so that the exit end portion has smaller minor diameters of internal thread than a middle portion, and thus forming a contracted internal thread portion having a conical section.
Alternatively, a hole may be formed so as to extend axially through an internal-thread member and, after that, the step of forming corrugation in the exit end portion and then the step of contracting may be performed. In accordance with such a method, in either case, continuous corrugation can be formed over one or more threads on ridges of the exit end portion of the internal thread portion, and a desired form of contraction can be readily achieved.
More specifically, the contracting step of forming the contracted thread portion may be performed as follows: The outer circumferential surface of the exit end portion of a material for the internal thread portion can be tapered so as to have outside diameters which are the smaller, the closer to the end side. A die having a tapered inner circumferential surface which is to engage with the tapered outer circumferential surface of the material for the internal thread portion can be pressed relatively against the tapered outer circumferential surface in the axial direction. With such tapered outer circumferential surface of the exit end portion of the internal-thread member, the exit end portion has the smaller thicknesses, the closer to the extremity thereof. Moreover, the internal thread portion of the exit end portion has continuous depressions over the entire circumference thereof. Accordingly, a process of squeezing inward can be readily performed to evenly contract the exit end portion.